1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus including a copying machine, a printer, and a facsimile machine. More specifically, the present invention relates to an image forming apparatus that adjusts a transfer voltage to be applied to a transfer member for transferring a toner image onto a transfer material.
2. Description of the Related Art
An electrophotographic type image forming apparatus employs the following method. That is, an image forming process, such as charging process, exposure process, and development process, is performed on a surface of an electrophotographic photosensitive member such as a photosensitive drum or a photosensitive belt to thereby process a target image into a visible toner image, followed by transferring the toner image onto a transfer material by transfer process.
Recently, a transfer member such as a transfer roller has been widely used in the transfer process. The transfer member is a part of a transfer unit that is pressed against the photosensitive member via a transfer material (e.g., an intermediate transfer member or a transfer belt), and electrostatically transfers the toner image on a surface of the photosensitive member onto the transfer material in the transfer unit by applying a voltage to the photosensitive member.
On the other hand, it is known that a resistance of the transfer member to be used in the transfer process changes depending on temperature and humidity under the ambient atmosphere and operating time. According to the change of resistance, an optimum transfer voltage value for obtaining a transfer image of a good quality always changes.
More specifically, degradation of a transfer property may occur if a voltage value lower than the optimum voltage value is applied as a transfer voltage value. Specifically, in the case of a multicolor image forming apparatus, degradation of a color stabilization property may occur due to the degradation of the transfer property through operating hours.
On the other hand, if a voltage value higher than the optimum voltage value is applied, an abnormal electrical discharge may occur in a transfer nip portion, resulting in image degradation caused by the abnormal electrical discharge. Therefore, it is preferable to adjust the transfer voltage value according to the change of resistance of the transfer member.
In view of the above, Japanese Patent Application Laid-open No. 2001-125338 discusses a constant voltage control method according to an “active transfer voltage control (ATVC) method”. More specifically, in the above method, a predetermined voltage value is applied to the transfer member before starting an image forming process and an output current value at the time is detected.
A resistance value between the transfer member and the photosensitive member is obtained based on the applied voltage and the detected current value, and the transfer voltage value to be applied to the transfer member at the time of forming a subsequent image is adjusted according to thus obtained resistance value.
On the other hand, an image forming apparatus, in which photodischarging (light neutralization) exposure process is performed on a surface of a photosensitive member after transfer process, is discussed in Japanese Patent Application Laid-open No. 60-147780. The photodischarging process can realize a good durability and has a high discharging property in comparison with discharging process in which a brush is brought into contact with an image carrier. That is, the photodischarging process is more advantageous than the discharging process using a brush.
According to the photodischarging exposure method, potential remaining on the surface of the photosensitive member generated between the transfer process and the charging process is discharged after the toner image is transferred onto the transfer member from the photosensitive member in the image forming process.
As a result of the above, uniform potential can be achieved for the photosensitive member before being charged for the next image forming process. Accordingly, the photosensitive member can be charged uniformly in the charging process for the next image forming process, and thereby the image degradation caused by a history of the residual charge is restrained.
Therefore, a method is widely used in which a photodischarging device such as a discharging LED is provided. The residual potential on the surface of the photosensitive member can be discharged, after a transfer process, by irradiating the surface of the photosensitive member with the photodischarging device after a transfer process.
However, a tandem type multicolor image forming apparatus, in which a plurality of image forming units, each including a photosensitive member, are arranged in parallel with a belt member which carries a recording material or a toner image, is reduced in size by reducing a distance between the adjacent photosensitive members. With the configuration described above, the distance between the adjacent photosensitive members becomes narrower. The photodischarging process causes the following problems if the photodischarging process is performed while the distance between the adjacent photosensitive members is made narrower.
Discharging light for discharging a first photosensitive member is initially irradiated onto a first electrophotographic photosensitive member, and, at the same time, is reflected by a surface of the first photosensitive member, a belt member, or the like.
The reflected discharging light (reflected light) is irradiated onto a second electrophotographic photosensitive member that is positioned at a downstream side of the first photosensitive member in a rotational direction of the belt member so as to be adjacent to the first photosensitive member. The reflected light is irradiated onto a region between a development unit of the second photosensitive member and a transfer unit.
At the time of forming an image, since a toner image is formed on the second photosensitive member in the above region, only a small adverse effect by the reflected light is exerted onto the image.
However, during the ATVC process, a process to form a toner image on the photosensitive member needs to be stopped. In order to shorten the stop time, a period of carrying out the ATVC process overlaps to each other between the adjacent image forming units in the image forming apparatus including a plurality of photosensitive members.
If the first photosensitive member is exposed to light from a discharging exposure device during the ATVC process, since no toner image is formed on the second photosensitive member during the ATVC process, potential of the photosensitive member in front of a transfer unit of the second photosensitive member varies because of the exposure to the reflected light.
If an amount of the reflected light is constant, no problem occurs when the image forming process is carried out at a transfer voltage set according to the ATVC process having been carried out in this situation. However, the state of the surface of the photosensitive member or the surface of the belt member changes according to the image forming process.
Therefore, if the ATVC process is carried out while there is the reflected light, an optimum transfer voltage may not be selected depending on a state of use of the image forming apparatus, which is not preferable.
A similar problem will also occur in a case where a transfer condition is set according to a process other than the ATVC process, which has been described above.
To solve the above problem, a shielding member may be provided in order to prevent the reflected light from irradiating. However, in the image forming apparatus having been downsized with a narrower distance between the adjacent photosensitive members, it is difficult to obtain a higher shielding property against the reflected light because there is only a limited space for installing the shielding member.